The present invention relates to an active resonant circuit designed to be incorporated in a fixed frequency microwave filter having a low insertion loss, a relative bandwidth equal at most to 10.sup..sup.-2 , which is designed for an input level of the order of 0 dBm, i.e. substantially 20 dB above that of filters commercially available at the present day.
The expression "microwave frequencies" refers to the frequencies ranging between 10.sup.8 and 2.10.sup.10 Hz. At the lower frequencies of this band, it is known to utilize semiconductors for loss compensation in filter circuits, in such manner that the attenuation be negligible in the bandwidth. The low frequency filters are not directly scalable at UHF and VHF frequencies due to the utilization of an active element, generally a transistor. The behaviour of the transistor at these higher frequencies leads to a rather complex equivalent circuit due to importance of stray impedances. Studies have been effected which have afforded solutions in some cases. U.S. Pat. No. 3,723,773, filed on the 27th May 1971, discloses a filter cell comprising a transistor connected in an inverted common collector circuit, and also two examples of filters the bandwidth of which is approximately 2% at 500 MHz. The main disadvantage of this embodiment consists in that this transistor connection forbids the application of an input level higher than approximately -20 dBm.
The present invention relates to pass-band filters having a relative bandwidth lower than 10.sup..sup.-2 and which are insensitive to temperature, capable of operating at a high input mean power up to one milliwatt. A given type of transistor used in a filter according to the invention may be operated at a level higher than that of known filters utilizing the same transistor.
The filters according to the invention have a bandwidth which is sufficiently free from temperature variation to provide operation thereof at a fixed frequency with a bandwidth lower than 1%. The active resonant circuits according to the invention have a thermal drift coefficient at most equal to 10.sup..sup.-5 per .degree.C and an insertion loss close to 0 dB at an input level up to 1 milliwatt.
A further advantage of the invention consists in that setting of the relative bandwidth is independent of the tuning adjustment. The relative bandwidth may be selected within wide limits which may range between 10.sup..sup.-2 and 10.sup..sup.-4. This is achieved, once the circuit is set, by adjusting the emitter current. This adjustment simultaneously provides for compensation of the dispersion of the characteristics of the transistors. The input level limit is fixed by the breakdown voltage of the transistor and, as a first approximation, by the transformation ratio of the input transformer. Thus, it may also be set independently of the resonance frequency.
It is possible to increase the stability of the resonance frequency relative to the temperature by matching of the temperature coefficients of the coupling components.